1. Field of the Invention
This invention relates to a photographic wide angle lens, and more particularly to a photographic wide angle lens system suitable for use in inexpensive panoramic photography.
2. Prior Art
Recently, lens-equipped photographic film packages or the so-called disposable photographic cameras are coming into wide use. Even the cameras of this sort are required to meet increasing demands for various functions. Among the inexpensive photographic cameras now on the market is a new model which permits panoramic photography without adding an optional mechanism for this purpose.
The cameras which are capable of such panoramic photography generally employ a camera construction of a larger scale involving complications in lens and other constructions, which will be eventually reflected by a higher cost and relatively difficult operations in use.
Under these circumstances, there has been introduced a camera for pseudo-panoramic photography, which is arranged to mask the upper and lower portions of frames of an ordinary 35 mm film or the like and to print the oblong picture of each partially masked frame on an enlarged scale.
As a wide angle photographic lens suitable for the inexpensive cameras of the sort mentioned above, there has been known the so-called hypergon lens consisting of two elements, more specifically, a perfectly symmetrical lens system having two identical lens elements located symmetrically on the front and rear sides of an aperture stop. This lens system has a feature that, by employing as lens elements a couple of landscape lenses or so-called meniscus camera lenses each with a concave surface disposed on the side of the aperture stop, it becomes possible to reduce coma, lateral chromatic aberration and distortion while flattening the tangential image plane. However, because of non-flattening of the spherical aberration and sagittal image plane, this lens system is incapable of making corrections to a sufficient degree for astigmatism which causes focusing off the tangential image plane, compelling to sacrifice the brightness and picture quality.
On the other hand, with a wider field angle, the field curvature becomes conspicuous. In this regard, for example, as disclosed in U.S. Pat. Nos. 3,006,248 and 4,932,764, attempts have been made to solve this problem by curving the film surface into a cylindrical shape in conformity with the Petzval image plane. However, this arrangement with a curved film surface lends itself to increase distortion considerably, impairing the feature of the hypergon lens which has less distortion for a flat image plane.
Further, the above-mentioned U.S. Pat. No. 4,932,764, as construed from its claim reading "front lens functions as a landscape lens and has a shorter focal length than said rear lens and said rear lens functions as a meniscus corrector", tries to solve the problem of distortion by balancing distortion with the Petzval field curvature which substantially coincides with the cylindrical film surface. It is gathered from the description in examples that, the ratio (fF/fR) of front lens focal length fF to rear lens focal length fR being about 0.61 to 0.82, the afore-mentioned drawback may be overcome. However, no sufficient improvement is made with regard to astigmatism which is another drawback of the two-element lens system. Accordingly, when the film surface is arranged to fit the sagittal image plane, it will not fit the tangential image plane. Further, even if the film surface is arranged to coincide with the petzval image plane taken as an average image plane, the resultant picture will suffer from degraded picture quality in peripheral regions as compared with center regions, failing to produce a picture which has high picture quality uniformly in overall regions of a picture.